Minimum stability

[LeoColomb]

Hi!

There is a mismatch in the composer.json which result to a composer update error.

The minimum stability

{
    "minimum-stability": "stable"
}

is not compatible with a dev-master non stable version

{
    "satooshi/php-coveralls": "dev-master"
}

Also, because stable is the default value of the minimum-stability (see docs) it shouldn't be rewrite to be more manageable.

👌